NFSD: Hoist rq_vec preparation into nfsd_read()

Accrue the following benefits:

a) Deduplicate this common bit of code.

b) Don't prepare rq_vec for NFSv2 and NFSv3 spliced reads, which
   don't use rq_vec. This is already the case for
   nfsd4_encode_read().

c) Eventually, converting NFSD's read path to use a bvec iterator
   will be simpler.

In the next patch, nfsd_iter_read() will replace nfsd_readv() for
all NFS versions.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs3proc.c

@@ -151,8 +151,6 @@ nfsd3_proc_read(struct svc_rqst *rqstp)
 {
 	struct nfsd3_readargs *argp = rqstp->rq_argp;
 	struct nfsd3_readres *resp = rqstp->rq_resp;
-	unsigned int len;
-	int v;
 
 	dprintk("nfsd: READ(3) %s %lu bytes at %Lu\n",
 				SVCFH_fmt(&argp->fh),
@@ -166,17 +164,7 @@ nfsd3_proc_read(struct svc_rqst *rqstp)
 	if (argp->offset + argp->count > (u64)OFFSET_MAX)
 		argp->count = (u64)OFFSET_MAX - argp->offset;
 
-	v = 0;
-	len = argp->count;
 	resp->pages = rqstp->rq_next_page;
-	while (len > 0) {
-		struct page *page = *(rqstp->rq_next_page++);
-
-		rqstp->rq_vec[v].iov_base = page_address(page);
-		rqstp->rq_vec[v].iov_len = min_t(unsigned int, len, PAGE_SIZE);
-		len -= rqstp->rq_vec[v].iov_len;
-		v++;
-	}
 
 	/* Obtain buffer pointer for payload.
 	 * 1 (status) + 22 (post_op_attr) + 1 (count) + 1 (eof)
@@ -187,7 +175,7 @@ nfsd3_proc_read(struct svc_rqst *rqstp)
 
 	fh_copy(&resp->fh, &argp->fh);
 	resp->status = nfsd_read(rqstp, &resp->fh, argp->offset,
-				 rqstp->rq_vec, v, &resp->count, &resp->eof);
+				 &resp->count, &resp->eof);
 	return rpc_success;
 }
 

fs/nfsd/nfsproc.c

@@ -176,9 +176,7 @@ nfsd_proc_read(struct svc_rqst *rqstp)
 {
 	struct nfsd_readargs *argp = rqstp->rq_argp;
 	struct nfsd_readres *resp = rqstp->rq_resp;
-	unsigned int len;
 	u32 eof;
-	int v;
 
 	dprintk("nfsd: READ    %s %d bytes at %d\n",
 		SVCFH_fmt(&argp->fh),
@@ -187,17 +185,7 @@ nfsd_proc_read(struct svc_rqst *rqstp)
 	argp->count = min_t(u32, argp->count, NFSSVC_MAXBLKSIZE_V2);
 	argp->count = min_t(u32, argp->count, rqstp->rq_res.buflen);
 
-	v = 0;
-	len = argp->count;
 	resp->pages = rqstp->rq_next_page;
-	while (len > 0) {
-		struct page *page = *(rqstp->rq_next_page++);
-
-		rqstp->rq_vec[v].iov_base = page_address(page);
-		rqstp->rq_vec[v].iov_len = min_t(unsigned int, len, PAGE_SIZE);
-		len -= rqstp->rq_vec[v].iov_len;
-		v++;
-	}
 
 	/* Obtain buffer pointer for payload. 19 is 1 word for
 	 * status, 17 words for fattr, and 1 word for the byte count.
@@ -207,7 +195,7 @@ nfsd_proc_read(struct svc_rqst *rqstp)
 	resp->count = argp->count;
 	fh_copy(&resp->fh, &argp->fh);
 	resp->status = nfsd_read(rqstp, &resp->fh, argp->offset,
-				 rqstp->rq_vec, v, &resp->count, &eof);
+				 &resp->count, &eof);
 	if (resp->status == nfs_ok)
 		resp->status = fh_getattr(&resp->fh, &resp->stat);
 	else if (resp->status == nfserr_jukebox)

fs/nfsd/vfs.c

@@ -1036,6 +1036,50 @@ __be32 nfsd_readv(struct svc_rqst *rqstp, struct svc_fh *fhp,
 	return nfsd_finish_read(rqstp, fhp, file, offset, count, eof, host_err);
 }
 
+/**
+ * nfsd_iter_read - Perform a VFS read using an iterator
+ * @rqstp: RPC transaction context
+ * @fhp: file handle of file to be read
+ * @file: opened struct file of file to be read
+ * @offset: starting byte offset
+ * @count: IN: requested number of bytes; OUT: number of bytes read
+ * @base: offset in first page of read buffer
+ * @eof: OUT: set non-zero if operation reached the end of the file
+ *
+ * Some filesystems or situations cannot use nfsd_splice_read. This
+ * function is the slightly less-performant fallback for those cases.
+ *
+ * Returns nfs_ok on success, otherwise an nfserr stat value is
+ * returned.
+ */
+__be32 nfsd_iter_read(struct svc_rqst *rqstp, struct svc_fh *fhp,
+		      struct file *file, loff_t offset, unsigned long *count,
+		      unsigned int base, u32 *eof)
+{
+	unsigned long v, total;
+	struct iov_iter iter;
+	loff_t ppos = offset;
+	struct page *page;
+	ssize_t host_err;
+
+	v = 0;
+	total = *count;
+	while (total) {
+		page = *(rqstp->rq_next_page++);
+		rqstp->rq_vec[v].iov_base = page_address(page) + base;
+		rqstp->rq_vec[v].iov_len = min_t(size_t, total, PAGE_SIZE - base);
+		total -= rqstp->rq_vec[v].iov_len;
+		++v;
+		base = 0;
+	}
+	WARN_ON_ONCE(v > ARRAY_SIZE(rqstp->rq_vec));
+
+	trace_nfsd_read_vector(rqstp, fhp, offset, *count);
+	iov_iter_kvec(&iter, ITER_DEST, rqstp->rq_vec, v, *count);
+	host_err = vfs_iter_read(file, &iter, &ppos, 0);
+	return nfsd_finish_read(rqstp, fhp, file, offset, count, eof, host_err);
+}
+
 /*
  * Gathered writes: If another process is currently writing to the file,
  * there's a high chance this is another nfsd (triggered by a bulk write
@@ -1161,14 +1205,24 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
 	return nfserr;
 }
 
-/*
- * Read data from a file. count must contain the requested read count
- * on entry. On return, *count contains the number of bytes actually read.
+/**
+ * nfsd_read - Read data from a file
+ * @rqstp: RPC transaction context
+ * @fhp: file handle of file to be read
+ * @offset: starting byte offset
+ * @count: IN: requested number of bytes; OUT: number of bytes read
+ * @eof: OUT: set non-zero if operation reached the end of the file
+ *
+ * The caller must verify that there is enough space in @rqstp.rq_res
+ * to perform this operation.
+ *
  * N.B. After this call fhp needs an fh_put
+ *
+ * Returns nfs_ok on success, otherwise an nfserr stat value is
+ * returned.
  */
 __be32 nfsd_read(struct svc_rqst *rqstp, struct svc_fh *fhp,
-	loff_t offset, struct kvec *vec, int vlen, unsigned long *count,
-	u32 *eof)
+		 loff_t offset, unsigned long *count, u32 *eof)
 {
 	struct nfsd_file	*nf;
 	struct file *file;
@@ -1183,12 +1237,10 @@ __be32 nfsd_read(struct svc_rqst *rqstp, struct svc_fh *fhp,
 	if (file->f_op->splice_read && test_bit(RQ_SPLICE_OK, &rqstp->rq_flags))
 		err = nfsd_splice_read(rqstp, fhp, file, offset, count, eof);
 	else
-		err = nfsd_readv(rqstp, fhp, file, offset, vec, vlen, count, eof);
+		err = nfsd_iter_read(rqstp, fhp, file, offset, count, 0, eof);
 
 	nfsd_file_put(nf);
-
 	trace_nfsd_read_done(rqstp, fhp, offset, *count);
-
 	return err;
 }
 

fs/nfsd/vfs.h

@@ -115,8 +115,12 @@ __be32		nfsd_readv(struct svc_rqst *rqstp, struct svc_fh *fhp,
 				struct kvec *vec, int vlen,
 				unsigned long *count,
 				u32 *eof);
-__be32 		nfsd_read(struct svc_rqst *, struct svc_fh *,
-				loff_t, struct kvec *, int, unsigned long *,
+__be32		nfsd_iter_read(struct svc_rqst *rqstp, struct svc_fh *fhp,
+				struct file *file, loff_t offset,
+				unsigned long *count, unsigned int base,
+				u32 *eof);
+__be32		nfsd_read(struct svc_rqst *rqstp, struct svc_fh *fhp,
+				loff_t offset, unsigned long *count,
 				u32 *eof);
 __be32 		nfsd_write(struct svc_rqst *, struct svc_fh *, loff_t,
 				struct kvec *, int, unsigned long *,